There have been proposed a variety of photovoltaic elements such as solar cell and power source for commercial and home appliances. They utilize the pn junction formed by ion implantation or thermal diffusion of impurities into a substrate of single crystal of silicon (Si) or gallium arsenide (GaAs), or by epitaxial growth of an impurity-doped layer on a substrate of such single crystal. However, there is a disadvantage for these photovoltaic elements that their production cost unavoidably becomes costly because of using a single crystal substrate. Because of this, they have not yet gained general acceptance for use as solar cell or as power source in commercial and home appliances.
Recently, there has been proposed a photovoltaic element in which there is utilized pin junction of amorphous silicon (hereinafter referred to as "A-Si") deposited film formed on an inexpensive substrate of non-single crystal material such as glass, metal, ceramics, and synthetic resin by way of the glow discharge decomposition method. This photovoltaic element has a nearly satisfactory performance and is of low production cost and because of this, it has been recognized usable as power source for some kinds of appliances such as electronic calculators and wrist watches.
However, for this photovoltaic element, there is a disadvantage that the output voltage is low because the band gap of the A-Si film constituting the element is 1.7 eV, which is not large enough. There is another disadvantage that its photoelectric conversion efficiency is low for a light source such as fluorescent light which contains more short-wavelength light, so that its application is limited to appliances with very small power consumption.
There is a further disadvantage for said photovoltaic element that the constituent A-Si film is often accompanied with a character of the so-called Staebler-Wronsk, effect, with which the film being deteriorated upon continuous irradiation with intense light for a long period of time. In view of the above, the foregoing proposed photovoltaic element is not practical for use as a power solar cell for which it is required to stably and continuously exhibit the desired characteristics over a long period of time.
On the other hand, there have been proposed direct transition-type semiconductor films having a wide band gap, such as ZnSe (having a band gap of 2.67 eV) and ZnTe (having a band gap of 2.26 eV) and mixed crystal thereof ZnSe.sub.1-x Te.sub.x (where 0&lt;x&lt;1). And the public attention has been forcused on these semiconductor films. These semiconductor films are, in general, such that are formed on a substrate of single crystal by way of epitaxial growth. The as-grown film of ZnSe exhibits the n-type conductivity and the as-grown film of ZnTe exhibits the p-type conductivity. However for any of these films, it is generally recognized that it is difficult for the film to be controlled to the reverse conductivity. Further, in order to carry out the epitaxial growth upon the film formation, it is required to use a specific substrate of single crystal and to maintain the substrate at elevated temperature. And in this film formation, the deposition rate is low. Because of this, it is impossible to perform epitaxial growth on a commercially available substrate which is inexpensive and low heat-resistant such as glass and synthetic resin. These factors make it difficult to develop practically applicable semiconductor films using the foregoing commercially available substrates.
Even in the case where a semiconductor film should be fortunately formed on such commercially available substrate, the film will be such that is usable only in very limited applications.
In fact, there have been various proposals to form a direct transition-type semiconductor film on a non-single crystal substrate such as glass, metal, ceramics and synthetic resin. However, under any of such proposals, it is difficult to obtain a desired direct transition-type semiconductor film having satisfactory electrical characteristics because the resulting film becomes to be accompanied with defects of various kinds which make the film poor in electrical characteristics and on account of this, it is difficult for the film to be controlled with the film by doping it with an impurity.
In the meantime, amorphous film composed of Zn and Se elements is described in U.S. Pat. No. 4,217,374 (called "literature 1" hereinafter) and also in U.S. Pat. No. 4,226,898 (called "literature 2" hereinafter). And ZnSe compound is described in Japanese Patent Laid-open No. 189649/1986 (called "literature 3" hereinafter) and Japanese Patent Laid-open No. 189650/1986 (called "literature 4" hereinafter).
Now, literature 1 discloses amorphous semiconductor films containing selenium (Se), zinc (Zn), hydrogen (H) and lithium (Li); but the subject lies in amorphous selenium, semiconductor films and the Zn described therein is merely an additive as well as Li and H. And as for the Zn and the Li, likewise in the case of the H, they are used aiming at reduction of the local state density in the energy gap without changing the inherent characteristics of the film. In other words, the addition of Zn to the amorphous Se film mentioned in literature 1 is not intended to positively form a ZnSe compound. Incidentally, literature 1 mentions nothing about the ZnSe compound and the formation of ZnSe crystal grains. Regarding the addition of Li, it should be noted that it is not added as a dopant.
Literature 2 does mention amorphous semiconductor films containing Se, Zn, and H. However, it deals mainly with amorphous silicon, and it defines Se as an element to form a compound with said silicon. As for the Zn, it defines as an element to sensitize the photoconductivity and reduce the local state density in the energy gap. In other words, the additions of Zn and Se are not intended to form a ZnSe compound. Incidentally, literature 2 mentions nothing about the ZnSe compound and the formation of ZnSe crystal grains.
Literature 3 and literature 4 are concerned with the deposition of a ZnSe film by HR-CVD method (hydrogen radical assisted CVD method). That is, they disclose methods of improving the deposition rate and the productivity of a deposited film; but they merely mention deposited films of non-doped ZnSe.
Against these backgrounds, there is an increased social demand to provide an inexpensive photovoltaic element having a high photoelectric conversion efficiency which may be practically usable as solar cell and also as a power source in various appliances.